1. Field of the Invention
The invention relates generally to photography, and particularly to a motor-driven film transport apparatus for a photographic camera.
2. Description of the Prior Art
Generally speaking, commercially available 35 mm film cassettes comprise a hollow cylindrical shell having an integral throat or lipped portion which extends tangentially from the shell. A film spool on which the filmstrip is wound is freely rotatable within the cassette shell. The filmstrip has a leading end section, commonly referred to as a "film leader", which protrudes from a light-trapped slit in the throat portion to the outside of the cassette shell. Typically, the protruding leader is 2-3 inches in length.
Commonly assigned U.S. Pat. No. Reissue 32,558, granted Dec. 15, 1987, discloses an auto-threading camera to be used with a conventional 35 mm film cassette, wherein a single drive motor via suitable gearing rotates a take-up drum at a faster speed than it rotates a metering sprocket. Initially, the sprocket is rotated by the motor to thread the film leader to the faster-moving drum. Once the film leader is secured to the drum, a unidirectional ratchet clutch for rotating the sprocket allows the sprocket to be overdriven by the filmstrip, whereupon the sprocket serves its normal function of acting as a film-driven frame sensor. During rewind of the filmstrip back into the cassette shell after exposure is completed, the clutch disconnects the sprocket from the motor, allowing the sprocket to continue to be film-driven In addition, the film spool within the cassette shell is rotated by the motor to rewind the filmstrip onto the spool, and the drum is rotated by the motor at a faster speed to facilitate disengagement of the filmstrip from the drum.
In conventional 35 mm film cassettes, one end of the film spool on which the filmstrip is wound has a short axial extension which projects from the cassette shell, enabling the spool to be turned by hand. If the spool is initially rotated in an unwinding direction, the film convolutions inside the cassette shell will tend to expand radially since the inner end of the filmstrip is attached to the spool, and the film leader protruding from the slit will remain stationary. The film convolutions can expand radially until a non-slipping relation is established between the outermost convolution and the inner curved wall of the cassette shell. Once this non-slipped relation exists, there is a binding effect between the outermost convolution and the shell which prevents further rotation of the spool in unwinding direction. Thus, rotation of the spool in the unwinding direction cannot serve to advance the filmstrip out of the shell, and it is necessary in the typical 35 mm camera to engage the protruding leader as in U.S. Pat. No. Reissue 32,558 to draw the filmstrip out of the shell.
A 35 mm film cassette has been proposed which, unlike conventional film cassettes, includes a film leader that does not extend outside the cassette shell. The film leader, instead, is located entirely within the cassette shell. Specifically, in U.S. Pat. No. 4,423,943, granted Jan. 3,1984, there is disclosed a film cassette wherein the outermost convolution of the film roll round on the film spool is radially constrained by respective circumferential lips of two axially spaced flanges of the spool to prevent the outermost convolution from contacting an inner wall of the cassette shell. The trailing end of the filmstrip is secured to the film spool, and the leading end of the filmstrip is reduced in width to allow it to protrude from between the circumferential lips and rest against the shell wall. During unwinding rotation of the film spool, the leading end of the filmstrip is advanced to and through a film passageway opening in order to exit the cassette shell. The opening has a width which is slightly less than the width of the filmstrip, thus resulting in the filmstrip being transversely bowed as it is uncoiled from the film spool, and thereby facilitating movement of the film edges under the circumferential lips of the respective flanges.
While the film cassette disclosed in U.S. Pat. No. 4,423,943 includes a non-protruding film leader which is automatically advanced to the outside of the cassette shell when the film spool is rotated in the unwinding direction, the film cassette appears to be intended to be loaded in a camera only after the film leader is advanced to protrude from the cassette shell. In the patent, it is suggested that one manually rotate the film spool relative to the cassette shell until the leader can be manually grasped and attached to a film advancing device in a camera.